Gallium compounds, including gallium nitrate, gallium sulfate, and gallium maltolate, have been repeatedly shown to have anti-inflammatory activities when administered systemically (i.e., orally, intravenously, or by other means that introduce gallium into the bloodstream and allow for its distribution through the body). Particular efficacy for gallium has been reported in animal models of rheumatoid arthritis (Delbarre F, Rabaud M, COMPTES RENDUS DE L'ACADÉMIE DES SCIENCES, SERIES D 283:1469-1472, 1976; Matkovic V et al., CURRENT THERAPEUTIC RESEARCH 50:255-267, 1991; U.S. Pat. No. 5,175,006 to Matkovic et al.), multiple sclerosis (Whitacre C et al., JOURNAL OF NEUROIMMUNOLOGY 39:175-182, 1992), uveitis (Lobanoff M C et al., EXPERIMENTAL EYE RESEARCH 65:797-801, 1997), and Type 1 diabetes (Flynn J O et al., DIABETES 41:38A, 1992). Systemically administered gallium has also shown efficacy in the treatment of cancer and infectious disease (Bernstein L R, PHARMACOLOGICAL REVIEWS 50:665-682, 1998). Locally administered gallium is effective in treating psoriasis and related dermatologic disorders (U.S. Pat. No. 5,747,482 to Bernstein). It has now been surprisingly discovered that locally administered gallium can reduce pain, itching, allodynia, hyperalgesia, and related symptoms.
Locally administered gallium is particularly effective in relieving neuropathic symptoms, especially peripheral neuropathic pain. Peripheral neuropathic pain is apparently caused by damage to peripheral neurons, and is typically characterized as “burning,” “shooting,” “stabbing,” or “electric-shock-like.” It may occur without external stimulation or, very commonly, it may be manifested as allodynia (an experience of pain from normally non-painful stimuli, such as from light touching) or hyperalgesia (an exaggerated sense of pain from a normally painful stimulation). The pain can be very intense and disabling; the pain from trigeminal neuralgia (neuropathy of the trigeminal nerve) is considered among the most severe types of pain known.
At least two million adults are estimated to have neuropathic pain in the United States, with the great majority having peripheral rather than central neuropathic pain (Morely-Forster P, PAIN RESEARCH MANAGEMENT 11 Suppl A:5A-10A, 2006). Some of the most common causes of peripheral neuropathic pain are diabetes, HIV infection, postherpetic neuralgia, trigeminal neuralgia, cancer, and cancer treatments. Numerous other causes of peripheral neuropathic pain are also known, including trauma, non-HIV infections, drugs, toxins, surgery, and complex regional pain syndrome (also called causalgia or reflex sympathetic dystrophy syndrome). The etiology for many cases of peripheral neuropathic pain is never discovered. Some of the common causes of peripheral neuropathic pain are summarized below.
Painful Diabetic Neuropathy:
Diabetes afflicts about 21 million people in the United States (National Institute of Diabetes and Digestive and Kidney Diseases, NATIONAL DIABETES STATISTICS FACT SHEET: GENERAL INFORMATION AND NATIONAL ESTIMATES ON DIABETES IN THE UNITED STATES, 2005) and nearly 200 million people worldwide (Wild S et al., DIABETES CARE 27:1047-1053, 2004). Painful diabetic neuropathy is estimated to affect approximately 20-24% of diabetics, with pain being defined as a recording by the patient of at least 10 mm on a 100 mm visual analog pain scale (Schmader K E, THE CLINICAL JOURNAL OF PAIN 18:350-354, 2002). The pain occurs most commonly in the extremities.
Cancer and Cancer-Associated Iatrogenic Neuropathic Pain:
Neuropathic pain is estimated to afflict about a third of cancer patients (Davis M P, Walsh D, AMERICAN JOURNAL OF HOSPICE PALLIATIVE CARE 21:137-142, 2004). In most cases the neuropathic pain is due to tumor tissue infiltrating or pressing on neurons, with other causes including nerve damage caused by surgery, chemotherapy, or radiotherapy. Treatments for breast cancer are particularly likely to cause neuropathic pain: nearly 50% of patients experience chronic pain following surgery for breast cancer, with most of the pain being neuropathic (Morely-Forster P, PAIN RESEARCH MANAGEMENT 11 Suppl A:5A-10A, 2006).
HIV-Infection Related Neuropathic Pain:
Neuropathic pain, generally due to distal sensory polyneuropathy, is estimated to afflict at least a third of those infected with HIV (Luciano C A et al., CURRENT OPINION IN NEUROLOGY 16:403-409, 2003). The cause is not always known, but may be due to HIV infection of neurons, the release of neurotoxins by macrophages, toxic reactions to drugs, opportunistic infections, or nutrient deficiencies. Neuropathic pain associated with HIV infection appears to be an under-recognized and under-treated condition.
Postherpetic Neuralgia:
Herpes zoster infection (shingles) is estimated to strike about 800,000 people each year in the United States (Schmader K E, THE CLINICAL JOURNAL OF PAIN 18:350-354, 2002). Pain associated with shingles is itself neuropathic, at least in part. The incidence of subsequent postherpetic neuralgia (nearly always a peripheral neuropathy) is directly correlated with age and with the severity of the herpetic rash. For herpes zoster patients over 50 years old (the great majority of herpes zoster patients), postherpetic neuralgia occurs in 50-68% one month after rash healing, in 25-50% three months after rash healing, and in 15-35% six months after rash healing (the lower numbers being for those treated with antiviral drugs) (Schmader K E, THE CLINICAL JOURNAL OF PAIN 18:350-354, 2002).
Peripheral neuropathic pain is clearly a widespread medical problem, occurring in millions of people worldwide, resulting from a wide range of causes.
Current Treatments for Peripheral Neuropathic Pain
Current preferred therapies for peripheral neuropathic pain include systemic antidepressants (particularly tricyclic antidepressants), anticonvulsants (including carbamazepine and gabapentin), opioid analgesics (including oxycodone, methadone, and dextromethorphan), and topical lidocaine and capsaicin. Few of these treatments produce even moderate pain relief for half the patients receiving them.
Antidepressants:
Antidepressants are commonly the first choice for treating neuropathic pain. A recent survey of the literature (Saarto T, Wiffen P J, THE COCHRANE DATABASE OF SYSTEMATIC REVIEWS 2005(3):CD005454, 2005) found that tricyclic antidepressants were the most effective, with amitriptyline being particularly effective. Amitriptyline, however, had an NNT (number needed to treat) value of 2 for moderate pain relief (that is, only 50% of those treated had at least moderate pain relief). In general, moderate pain relief for peripheral neuropathic pain was produced by tricyclic antidepressants in about 33-50% of patients, by serotonin noradrenaline reuptake inhibitors in about 20-25% of patients, and by selective serotonin reuptake inhibitors in about 14% of patients (Sindrup S H et al., BASIC & CLINICAL PHARMACOLOGY & TOXICOLOGY 96:399-409, 2005).
Anticonvulsants:
A number of anticonvulsants have been administered for the treatment of peripheral neuropathic pain, with gabapentin, carbamazepine, and phenytoin appearing to be the most effective. Again, however, the efficacy rates are fairly low. For moderate pain relief, gabapentin had an NNT of 3.2 in postherpetic neuralgia; in painful diabetic neuropathy it had an NNT of 3.8, while carbamazepine had an NNT of 2.3 and phenytoin had an NNT of 2.1 (Wiffen P et al., THE COCHRANE DATABASE OF SYSTEMATIC REVIEWS 2005 (3):CD001133, 2005). Other tested anticonvulsants appeared to be less effective. It is noted that anticonvulsants, as tricyclic antidepressants, frequently produce significant adverse effects in patients.
Opioid Analgesics:
A recent meta-analysis of the efficacy and safety of opioid agonists in the treatment of non-malignant neuropathic pain showed marginal to no efficacy in short-term (24 hour) studies, and marginal efficacy in intermediate-term (8-56 day) studies (Eisenberg E et al., JAMA 293:3043-3052, 2005). The drugs studied were morphine, oxycodone, methadone, and levorphanol. A study of levorphanol found that patients with postherpetic neuralgia had an average 14% reduction in pain with a low dose, and a 33% reduction with a high dose, though 31% of the subjects dropped out of the study due to drug side effects (Rowbotham M C et al., NEW ENGLAND JOURNAL OF MEDICINE 348:1223-1232, 2003). All the opioid drugs commonly produced significant but non-life-threatening adverse effects.
Topical Lidocaine:
A topically applied patch containing 5% lidocaine is commonly used to treat localized peripheral neuropathic pain. Clinical studies have shown that this treatment results in modest reductions of pain for many patients (e.g., Argoff C E et al., CURRENT MEDICAL RESEARCH AND OPINION 20 Suppl 2:S21-S28, 2004). One controlled study found an NNT of 2 in postherpetic neuralgia (Hempenstall K et al., PLoS MEDICINE 2:e164, 2005). To remain effective, the patches must be changed several times per day. The patches cause numbness of the contacted skin, and commonly cause skin irritation, and usually do not relieve severe pain.
Topical Capsaicin:
An analysis of two studies on the use of topical capsaicin cream to treat postherpetic neuralgia found a low efficacy rate, with an NNT value of 3.26 (Hempenstall K et al., PLoS MEDICINE 2:e164, 2005). It is noted that placebo-controlled studies with capsaicin are compromised due to the clearly noticeable sensations (including pain) produced in the skin by capsaicin; thus, perceived efficacy of capsaicin may benefit from a placebo effect.
Many other systemic and local treatments are used in attempts to relieve peripheral neuropathic pain, generally with no more than moderate success. Recently, subcutaneously injected botulinum-A toxin has been tried in a small number of patients with trigeminal neuralgia (Piovesan E J et al., NEUROLOGY 66:1458-1459, 2006) and in a single patient with postherpetic neuralgia (Liu H T et al., PAIN MEDICINE 7:89-91, 2006). The patient with postherpetic neuralgia reported pain relief for several weeks after numerous injections, followed by a recurrence of pain at pre-treatment levels. The patients with trigeminal neuralgia reported transient low to moderate pain relief following multiple injections, with side effects including muscle weakness.
It is thus apparent that currently available treatments for peripheral neuropathic pain have only low to moderate efficacy, and many patients are left without significant pain relief. The lack of adequate pain relief for millions of people with peripheral neuropathic pain, as well as for those with other types of pain, represents a great unmet medical need.